Obtaining high output power of laser radiation while simultaneously having low divergence of a laser beam may be an issue when creating a laser system.
The divergence of output laser radiation is mainly determined by the properties of laser cavity. Unstable cavities may be used for producing laser beams with very low divergence. In such cavities, light photons emitted by a laser medium reach the edge of cavity aperture in a fixed number of round trips even if the light photons were emitted under insignificant angles to the optical axis. In this manner, optical aberrations possibly existing in laser medium and cavity reflectors do not have a significant influence on optical quality of the output radiation.
Unstable cavities can be characterized by a magnification field factor M which may be defined as the ratio between sizes of a linear light beam in two subsequent trips in the cavity.
A laser based on an unstable cavity may comprise an output mirror with high reflectivity. In such a laser, an output laser beam may be produced beyond the aperture of the output mirror in a form of a ring. Such form of a light beam may, however, be ill-suited for applications requiring use of light beams with a maximum axial intensity (Gaussian spatial intensity distribution or close to uniform one).
Alternatively, a laser based on unstable cavities may comprise an output coupler which is semi-transparent. A compromise between the spatial divergence of the output beam and the convenience of its practical use may be achieved in such a laser.
Laser pulses with high peak power may be obtained, for example, by a method of Q-switching, also known as giant pulse formation. This laser operation mode may consist in changing cavity losses from a high value to a low value during excitation of a laser medium inside a laser cavity.
At the high value of cavity loss factor, a laser generation threshold remains at a level that prevents the development of laser generation. At this stage, the laser medium receives excitation energy from an excitation source and accumulates the energy. When a desired level of the stored energy is reached, the cavity losses are lowered to the low value as fast as possible. The generation threshold reduces dramatically and the laser begins to generate stimulated emission in a manner that the gain significantly exceeds the losses.
As a result, a powerful laser output pulse appears where the energy of the pulse is approximately the energy stored in the laser medium up to the moment of reduction of cavity losses. Pulse duration is equal to several periods of cavity trips.
Q-switching may be implemented in various ways. A simple approach is so-called opto-mechanical technique. In this approach, a high level of losses in the cavity is achieved by placing a mechanical chopper inside the cavity. The low level of losses is obtained by removing the chopper from the path of the radiation.
When the opto-mechanical technique is used, the time required for a transition from a high-loss state to a low-loss state can be minimized by using as small size of laser beam as possible in the plane of mechanical chopper position.
In order to minimize the laser beam size in the plane of mechanical chopper position, focusing properties of additional intracavity components or concave cavity mirrors may be used (see <<Scaling of a Q-switch CO2 laser for pulsed laser deposition>>, Proc. SPIE, Vol. 3343, 1998, pp.759-768).
For lasers with low pulse energy, the intracavity focusing of the radiction may be quite acceptable. However, in high-power laser systems, this may lead to undesirable effects, such as an optical breakdown. Duration of the pulse generation in Q-switched lasers can range from a few nanoseconds to a few hundred nanoseconds. Thus, the duration of the pulse generation may be much shorter than the time taken by mechanical means to reduce intracavity losses (few microseconds). The emission may reach its peak power while the mechanical chopper is still on the line of propagation of the emitted radiation. As a result, the mechanical chopper may be damaged by the emission.
Q-switching may also be performed, for example, by electro-optical technique. This type of Q-switching may be implemented by an optical element in which transmission depends on the electrical voltage applied to the element.
Implementation of electro-optical Q-switch technique into a high-power laser system may also be problematic as the electro-optical crystals generally have much lower optical breakdown threshold than other non-crystal intracavity components. For example, U.S. Pat. No. 4,498,179 discloses a pulsed laser comprising a laser medium placed in main cavity which output mirror is semi-transparent, an additional resonator optically coupled with said main cavity, electro-optical Q-switch inside said additional resonator, and a Q-switch control unit. The laser has a low output power of radiation because of the electro-optical modulator used.
A possible approach for creating a laser system that generates powerful pulses is a combination of low-power pulsed laser and an amplifier (<<Generation of CO2 laser pulses by Q-switching and cavity dumping and their amplification by a microwave excited CO2 laser>>, J. Phys. D: Appl. Phys. 29, 1996, pp.57-67). In such a system, a Q-switched laser generates a short pulse of low energy which is then used as an input pulse of the amplifier. As the input pulse passes once or more through the gain medium of amplifier, radiation reaches the required level of energy. In practice, however, laser systems comprising amplifiers may be complex and cumbersome. They may require use of additional synchronization systems which may lead to high costs and reduced efficiency.
Effective modulation of radiation in a main laser cavity can be provided either by placing a modulator inside the cavity, or by using an external radiation source for seeding modulated radiation into the main cavity which then plays a role of a multi-pass amplifier.
In the case of a modulator inside the laser cavity, requirements for Q-switching may be determined by the relation between the modulation depth and the lifetime of a photon in the cavity. In lasers where modulators with low modulation depth are used, generation of high-contrast pulses may be achieved only with low cavity losses. If the cavity losses are higher (and laser medium has high gain), the modulation depth may have to be increased as photons may have time to make only few round trips before leaving the cavity. Because of the low optical breakdown threshold of intracavity modulators, it may be difficult to simultaneously achieve a high peak power and a high average power of the laser radiation.
In the case of an external radiation source, the above restriction can be avoided but achieving high-contrast modulation of the radiation in the main cavity may become an issue. If an external modulated modulation seeding radiation is injected into a main cavity in which the conditions of self-excitation have already been reached and stimulated emission takes place independently of the seeding radiation, depth of the modulation of the output radiation in the main cavity depends on the power ratio between the seeding light and the self-excited emission. Thus, if the power of the seeding radiation is insufficient, the seeding radiation may not be able to impose its modulation to the emission of the main cavity. On the other hand, if the power of the modulated seeding radiation is significantly increased, the problems related to high power modulation as described above may arise.